The present invention relates to voltage regulator circuits and is directed more particularly to regulator circuits wherein the AC and DC output voltages are maintained at constant predetermined values by controlling the phase displacement between the input and output AC voltage waves.
Prior to the present invention, static circuits for providing a regulated AC output voltage have been of two major types. These include regulators of the ferroresonant type and switching regulators of the phase controlled type. In circuits of the former-type voltage regulation results from the action of a shunt tank circuit, including a capacitor and a saturable inductor, on a series inductor. Because the shunt capacitor must have a high capacitance value to support the load voltage during each half cycle thereof and because this capacitor must discharge and recharge with a reversed polarity twice during each AC cycle, circuit losses have been high resulting in poor efficiency. Additionally, since proper circuit operation is strongly dependent upon maintaining a predetermined relationship between the inductive and capactive reactances of the circuit, the loads to be used therewith had to be restricted to those having a high-power factor.
In switching regulators of the phase controlled type, regulation is achieved by controlling the conduction of a switch which is in series between the AC source and the AC load. If the input voltage is low, switching occurs early in each half cycle so that a large fraction of the input voltage appears across the AC load. If, on the other hand, the input voltage is high, switching occurs late in each half cycle to prevent much of the input voltage from appearing across the AC load. While regulator circuits of this type adequately control the magnitude of the desired AC voltage, they seriously distort the waveform of both the output voltage therefrom and the input current thereto. The latter distortion can detrimentally affect others using the same power line. In addition, because the switching elements are, in effect, connected across the line, regulator circuits of the phase controlled type are subject to misfiring in the presence of AC line voltage transients.